Mastering JavaScript Design Patterns - Second Edition

Mock

The final type of fake object is a mock. The difference between a mock and a stub is where the verification is done. With a stub, our test must check if the state is correct after the act. With a mock object, the responsibility for testing the asserts falls to the mock itself. Mocks are another place where it is useful to leverage a mocking library. We can, however, build the same sort of thing, simply, ourselves:

class MockCredentialFactory {
  constructor() {
    this.timesCalled = 0;
  }
  Create() {
    this.timesCalled++;
  }
  Verify() {
    assert(this.timesCalled == 3);
  }
}

This mockCredentialsFactory class takes on the responsibility of verifying the correct functions were called. This is a very simple sort of approach to mocking and can be used as such: ...